import torch
from abc import abstractmethod
from numpy import inf
from logger import TensorboardWriter


class BaseTrainer:
    """
    Base class for all trainers
    """

    def __init__(self, model, criterion, metric_ftns, optimizer, config):
        self.config = config
        self.logger = config.get_logger("trainer", config["trainer"]["verbosity"])

        # setup GPU device if available, move model into configured device
        self.device, device_ids = self._prepare_device(config["n_gpu"])
        self.model = model.to(self.device)
        if len(device_ids) > 1:
            self.model = torch.nn.DataParallel(model, device_ids=device_ids)

        self.criterion = criterion
        self.metric_ftns = metric_ftns
        self.optimizer = optimizer

        cfg_trainer = config["trainer"]
        self.epochs = cfg_trainer["epochs"]
        self.save_period = cfg_trainer["save_period"]
        self.monitor = cfg_trainer.get("monitor", "off")

        # configuration to monitor model performance and save best
        if self.monitor == "off":
            self.mnt_mode = "off"
            self.mnt_best = 0
        else:
            self.mnt_mode, self.mnt_metric = self.monitor.split()
            assert self.mnt_mode in ["min", "max"]

            self.mnt_best = inf if self.mnt_mode == "min" else -inf
            self.early_stop = cfg_trainer.get("early_stop", inf)

        self.start_epoch = 1

        self.checkpoint_dir = config.save_dir

        # setup visualization writer instance
        self.writer = TensorboardWriter(
            config.log_dir, self.logger, cfg_trainer["tensorboard"]
        )

        if config.resume is not None:
            self._resume_checkpoint(config.resume)

    @abstractmethod
    def _train_epoch(self, epoch):
        """
        Training logic for an epoch

        :param epoch: Current epoch number
        """
        raise NotImplementedError

    def train(self):
        """
        Full training logic
        """
        not_improved_count = 0
        for epoch in range(self.start_epoch, self.epochs + 1):
            result = self._train_epoch(epoch)

            # save logged informations into log dict
            log = {"epoch": epoch}
            log.update(result)

            # print logged informations to the screen
            for key, value in log.items():
                self.logger.info("    {:15s}: {}".format(str(key), value))

            # save model every period
            if epoch % self.save_period == 0:
                self._save_checkpoint(epoch)

            # evaluate model performance according to configured metric,
            # save best checkpoint as model_best.
            if self.mnt_mode != "off":
                try:
                    # check whether model performance improved or not, according to specified metric(mnt_metric)
                    improved = (
                        self.mnt_mode == "min" and log[self.mnt_metric] <= self.mnt_best
                    ) or (
                        self.mnt_mode == "max" and log[self.mnt_metric] >= self.mnt_best
                    )
                except KeyError:
                    self.logger.warning(
                        "Warning: Metric '{}' is not found. "
                        "Model performance monitoring is disabled.".format(
                            self.mnt_metric
                        )
                    )
                    self.mnt_mode = "off"
                    improved = False

                if improved:
                    self.mnt_best = log[self.mnt_metric]
                    not_improved_count = 0
                    # if best, directly save it.
                    self._save_checkpoint(epoch, save_best=True)
                else:
                    not_improved_count += 1

                if not_improved_count > self.early_stop:
                    self.logger.info(
                        "Validation performance didn't improve for {} epochs. "
                        "Training stops.".format(self.early_stop)
                    )
                    break

    def _prepare_device(self, n_gpu_use):
        """
        setup GPU device if available, move model into configured device
        """
        n_gpu = torch.cuda.device_count()
        if n_gpu_use > 0 and n_gpu == 0:
            self.logger.warning(
                "Warning: There's no GPU available on this machine,"
                "training will be performed on CPU."
            )
            n_gpu_use = 0
        if n_gpu_use > n_gpu:
            self.logger.warning(
                "Warning: The number of GPU's configured to use is {}, but only {} are available "
                "on this machine.".format(n_gpu_use, n_gpu)
            )
            n_gpu_use = n_gpu
        device = torch.device("cuda:0" if n_gpu_use > 0 else "cpu")
        list_ids = list(range(n_gpu_use))
        return device, list_ids

    def _save_checkpoint(self, epoch, save_best=False):
        """
        Saving checkpoints

        :param epoch: current epoch number
        :param log: logging information of the epoch
        :param save_best: if True, rename the saved checkpoint to 'model_best.pth'
        """
        arch = type(self.model).__name__
        state = {
            "arch": arch,
            "epoch": epoch,
            "state_dict": self.model.state_dict(),
            "optimizer": self.optimizer.state_dict(),
            "monitor_best": self.mnt_best,
            "config": self.config,
        }
        filename = str(self.checkpoint_dir / "checkpoint-epoch{}.pth".format(epoch))
        torch.save(state, filename)
        self.logger.info("Saving checkpoint: {} ...".format(filename))
        if save_best:
            best_path = str(self.checkpoint_dir / "model_best.pth")
            torch.save(state, best_path)
            self.logger.info("Saving current best: model_best.pth ...")

    def _resume_checkpoint(self, resume_path):
        """
        Resume from saved checkpoints

        :param resume_path: Checkpoint path to be resumed
        """
        resume_path = str(resume_path)
        self.logger.info("Loading checkpoint: {} ...".format(resume_path))
        checkpoint = torch.load(resume_path)
        self.start_epoch = checkpoint["epoch"] + 1
        self.mnt_best = checkpoint["monitor_best"]

        # load architecture params from checkpoint.
        if checkpoint["config"]["arch"] != self.config["arch"]:
            self.logger.warning(
                "Warning: Architecture configuration given in config file is different from that of "
                "checkpoint. This may yield an exception while state_dict is being loaded."
            )
        self.model.load_state_dict(checkpoint["state_dict"])

        # load optimizer state from checkpoint only when optimizer type is not changed.
        if (
            checkpoint["config"]["optimizer"]["type"]
            != self.config["optimizer"]["type"]
        ):
            self.logger.warning(
                "Warning: Optimizer type given in config file is different from that of checkpoint. "
                "Optimizer parameters not being resumed."
            )
        else:
            self.optimizer.load_state_dict(checkpoint["optimizer"])

        self.logger.info(
            "Checkpoint loaded. Resume training from epoch {}".format(self.start_epoch)
        )
